1. Field of the Invention
The present invention relates to a method for protecting high temperature alloys containing iron, nickel and chromium against high temperature corrosion caused by carburization or metal dusting.
2. Description of the Related Art
It is a major problem in many industrial processes that the high temperature alloys commonly used as construction materials are susceptible to corrosion by oxidation or carburization or metal dusting when exposed at high temperatures to gases with a high carbon potential. Carburization is observed in the petrochemical industry, where ethylene is produced in pyrolysis furnaces by thermal cracking of hydrocarbons in a steam-hydrocarbon mixture at temperatures up to 1100xc2x0 C. In this cracking process, coke deposition occurs at the inner walls of the cracking tubes. In steam reformers, natural gas or other hydrocarbons are converted by catalytic reaction on nickel catalysts to CO and H2. Carburization of the tube walls is observed after overheating or excessive carbon activities. In industrial furnaces for heat treatment or carburization of steels, carburization of the carrying grates and the furnace walls also occurs. Components of the CO2-cooled nuclear reactor may be carburized by CO2, and the heat exchangers of the helium-cooled reactor may be carburized by impurities such as CO and CH4 in the helium. In coal gasification and in waste incineration plants, carburization is possible but the sulphidation and corrosion by chlorine will be more severe. Downstream of the steam reforming furnace, the heat recovering equipment is potentially vulnerable to a severe form of corrosion known as xe2x80x9cmetal dustingxe2x80x9d. It is a catastrophic carburization process to which alloys containing iron, nickel and cobalt is vulnerable, which results in the disintegration of the alloy into xe2x80x9cdustxe2x80x9d consisting of particles of carbon, carbides, metal and oxides. The result is wastage of the alloy surface. In contrast to the above-mentioned carburization, metal dusting occurs at temperatures as low as approximately 450xc2x0 C. As a result of many studies, it has been concluded that virtually all available high temperature alloys are vulnerable to metal dusting. It has been shown that addition of H2S to the gas may provide some resistance towards carburization and metal dusting. However, because of the risk of undesirable effects, such as catalysts poisoning, this cannot be used in many cases. Efficient means, generally applicable, for protecting such alloys against high temperature corrosion have until now not been developed.
Usually, the protection of high temperature alloys against corrosion is dependent on the formation of an outer chromium-oxide layer. However, such an oxide layer may, under most practical conditions, not be protective for a very long time, because cracks can easily be formed in the oxide layer and spalling may occur due to loss of adherence to the underlying alloy. The same risks are present when a similar protection is attempted by coating the alloy surface with a protecting mixed oxide layer.
The method of the present invention does not suffer from such risks, because it does not depend on the formation of a surface oxide layer with thermal and mechanical properties vastly different from those of the alloy.
By the method of the present invention, a protective layer is formed on the surface of the high temperature alloy by annealing protection metals on the surface and thereby creating a thin surface alloy with the protective metal. Thus, the protective alloy has thermal and mechanical properties being similar to that of the high temperature alloy to be protected.
In accordance with the present invention, there is provided a method for protecting parts and components of industrial plants such as containers, tubes, ferrules, etc. made of high temperature alloys containing iron, nickel and chromium and/or aluminum against corrosion by carburization or metal dusting. The method comprises: (a) cleaning of the alloy surface, (b) deposition of a noble or precious metal or of an element from group IVA (i.e., Sn and Pb), and IVB, or from group VA (i.e., Sb and Bi) and VB on the surface, and (c) heating of the surface in an inert gas or in a gas mixture consisting of an inert gas, hydrogen and water vapor in such proportions that the gas mixture is reducing towards iron and nickel, but oxidizing towards chromium and aluminum at the temperature of the heat treatment.
The heating takes place at a predetermined temperature in the range of 800-1000xc2x0 C. for a period of time sufficient for the formation of a surface alloy consisting of the deposited element and one or more of the metallic elements of the substrate, high temperature alloy. It is conceivable that the formation of the stable surface alloy is decisive for the protection obtained.
Deposition of the above metals may be carried out by conventional methods including physical or chemical vapour deposition or dipping, spraying or plating. Preferably, the metal is deposited to a thickness in the range of 0.01 to 10 xcexcm.
Formed in this manner, the surface alloy is preferably a uniform distribution of the noble or precious metal or group IVA, IVB, VA or VB metal on and in the surface to be protected. For example, in cases where a gas mixture containing hydrogen and water vapor is used during the heating treatment, a thin chromium oxide and/or aluminum oxide layer is formed on top of the surface alloy. This thin oxide layer contributes to the protection of the alloy.